Information recording media (optical disks) which can record information only once with laser beams have hitherto been known. These information recording media are also called write once-type CDs (so-called CD-Rs), and have the advantage that a small amount of CDs can be provided at reasonable prices and promptly, compared with the production of the conventional CDs. With the recent spread of personal computers, demand for such CDs have increased. The CD-R type information recording media have the typical structure that a transparent disk-shaped substrate is laminated with a recording layer comprising an organic dye, a light reflective layer comprising a metal such as gold or silver, and a resin protective layer in this order. Information is recorded on the optical disk by irradiating a near infrared laser beam (usually, a laser beam having a wavelength of about 780 nm) to locally deform the recording layer by heat generation. On the other hand, reading (reproduction) of the information is usually carried out by irradiating a laser beam having the same wavelength as the laser beam for recording to detect the difference in reflectance between a deformed site (recorded area) of the recording layer by heat generation and a non-deformed site (non-recorded area).
In recent years, information recording media higher in recording density have been desired. In order to increase the recording density, it is effective to decrease the diameter of a laser beam irradiated. Further, it has been known a theory that a laser beam having a shorter wavelength is effective for an increase in density, because the diameter of the laser beam can be decreased. Accordingly, optical disks for carrying out recording and reproduction using laser beams having wavelengths shorter than 780 nm which has hitherto been used have been developed. For example, an optical disk called a write once-type digital versatile disk (so-called DVD-R) has been proposed. This optical disk is produced so as to provide the structure that two disks each comprising a transparent disk-shaped substrate having a diameter of 120 mm or 80 mm on which pre-grooves are formed at a track pitch of 0.7 to 0.8 μm which is narrower than 1.6 μm of the CD-R, having provided thereon a recording layer comprising a dye, and usually further a light reflective layer and a protective layer on the recording layer, or the disk and a disk-shaped protective substrate having substantially the same dimension as that of the disk are adhered to each other with an adhesive, facing the recording layer inside relative to the substrates. In the DVD-R, recording and reproduction are carried out by irradiation of a visible laser beam (usually, a laser beam having a wavelength ranging from 600 to 700 nm), and it is said that higher density recording than that of the CD-R is possible.
The DVD-R type information recording media can record information in an amount several times that of the CD-R type media. It is therefore desired that the probability of occurrence of errors in high-speed recording be small, particularly because of the necessity to rapidly process a large amount of information, not to mention that the DVD-R type media have high recording sensitivity. Further, the recording layers comprising the dyes are generally low in aging stability to heat or light, so that the development of recording layers which can maintain high performance against heat or light for a long period of time is desired.
Japanese Patent Laid-Open No. 209995/1988 discloses a CD-R type information recording medium comprising a substrate having provided thereon a recording layer comprising an oxonol dye. It is described that the use of the oxonol dye allows stable recording and reproduction characteristics to be maintained for a long period of time.
The present inventors applied the oxonol dyes described in Japanese Patent Laid-Open No. 209995/1988 to DVD-R type optical information recording media and studied their performance. The studies proved that the DVD-R type optical information recording media containing the oxonol dyes in recording layers are low in reflectance and modulation degree and are not sufficiently satisfactory in recording and reproduction characteristics. Further, the studies proved that the above-mentioned recording media are liable to develop poor reproduction in long-term exposure to light such as sunlight, so that they are insufficient in light resistance. On the other hand, Japanese Patent Laid-Open No. 138998/1999 discloses an optical information recording medium comprising a substrate having provided thereon a recording layer comprising an oxonol dye salt-bonded by a diimmonium which is a singlet oxygen quencher. It is described that the use of the oxonol dye allows stable recording and reproduction characteristics to be maintained even in long-term exposure to light such as sunlight. However, it has become clear that the optical information recording medium in which the recording layer is composed by the oxonol dye alone are liable to develop poor reproduction in long-term storage thereof under the circumstances of high temperature and humidity, and insufficient durability against heat and humidity.